This invention relates to an electronic postage scale and, more particularly, to such a scale having an improved automatic "zero" or reference level adjustment feature, and exhibiting simplified, improved automatic calibration. In an embodiment to be described, the electronic postage scale is provided with an electronic micro-processor unit for carrying out a parcel weighing operation, for calculating the required postage in accordance with the weighed parcel and a selected destination zone and class of shipment of that parcel, and for displaying pertinent information in accordance with the weighing and calculating operations.
In determining the requisite postage for a parcel to be mailed to a desired destination, such postage is dependent upon the weight of the parcel, the particular destination zone and the class of shipment that is desired. In general, each class of shipment which is available is associated with a unique postage rate. Thus, the United States Postal Service provides first, second and third class service, parcel post service, priority mail service, and book rate service. Also, United Parcel Service provides interstate, intrastate and blue label services.
Prior to the advent of automatic postage calculating devices, proper postage could be determined only by weighing the parcel to be shipped and then consulting the appropriate reference table associated with a selected class of shipment in order to find the postage for the weighed parcel which is to be shipped by that class of shipment to a desired destination zone. This postage-determining technique was time consuming and was subject to erros which inadvertently could occur during normal usage of the numerous reference tables; thus providing a definite need and desirability for automatic postage calculating devices.
One example of an automatic postage scale is disclosed in U.S. Pat. No. 3,738,438. Digital techniques are used to provide an automatic indication of the postage required for a selected class of shipment of a parcel to a destination zone. The postage data for each class of shipment and destination zone are provided as coded perforations in an adjustable postage code roll. Hence, the postage codes are arranged on the roll in a plurality of groups, each group representing the postage rates for a particular combination of destination zone and class of shipment. The roll is advanced until the proper rate group is selected, whereupon further codes representing the postage for various parcel weights in the selected rate group are sensed. The particular code associated with the weight of the parcel is read out and the postage data represented by that code is displayed.
The use of a coded roll in conjunction with a postage scale is both a laborious and time-consuming operation. Furthermore, the overall dimensions of the postage scale must be sufficiently large in order to accommodate suitable coded rolls; thus requiring that the postage scale occupy space that otherwise could be advantageously and economically used.
In the past, postage rates, and particularly U.S.P.S. rates, were not necessarily linear for various classes of shipment and destination zones. For example, the postage for a two-pound parcel shipped to a selected destination zone by a particular class of shipment did not necessarily include a factor which was twice the postage for a one-pound parcel. Therefore, in order to provide an automatic determination of proper postage, a recently proposed electronic postage scale used a plurality of electronic or magnetic memory units, or sections, each unit being associated with a particular class of shipment to store postage data for each increment of weight of a parcel to be shipped to each destination zone encompassed by that class of shipment. This type of postage scale is described in copending application No. 652,820, filed Jan. 27, 1976 now U.S. Pat. No. 4,051,913 by Gudea, a coinventor of the instant invention. While the postage scale of this copending application operates satisfactorily, the use of a large number of memory units contributes substantially to the overall cost of the scale. Furthermore, as postage rates for the various classes of shipment change, the associated memory units must be replaced by other units within which updated postage information is stored.
Recently, U.S.P.S. and UPS postage rates have been changed so as to be substantially linear for the various destination zones included in each class of shipment. With such linearity, it may not be necessary to store the postage data for each increment of weight of a parcel to be shipped to each destination zone via each class of shipment. Rather, the postage rate per unit of weight can be stored for each destination zone in each class of shipment, and the appropriate rate can be multiplied by the determined parcel weight in order to calculate the correct postage. This permits a significant reduction in memory capacity, thus correspondingly reducing the overall cost of the postage scale.
Another disadvantage of some previously proposed postage scales is that the zero, or reference level, against which a parcel is weighed can vary over a period of time. For example, in a scale of the type having various beams and balances coupled to and movable by the weighing platform, normal wear and tear of the mechanical components will result in an effective "drift" of the zero level. Hence, a parcel which actually weighs one pound three and one-half ounces, as an example, may be measured to weigh one pound three ounces or one pound four ounces. This change in the measured weight of the parcel will, of course, directly affect the calculated postage. Similarly, in electronic postage scales, changes in the parameters of the electrical components may occur due to temperature coefficients, longevity of the components, and the like. Also, the zero level may change because of accumulations of dust, dirt or other residue on the weighing platform. Heretofore, in order to account for such zero level changes, the postage scale must be periodically checked or examined for accuracy.
Still another disadvantage of some previously proposed postage scales resides in the calibration thereof. A typical calibration operation had been carried out by placing a standard weight on the weighing platform of the scale and then manually adjusting either various mechanical components or electrical elements (such as potentiometers, trimmers or the like) until the scale indicated the correct standard weight. Such calibration operations are time-consuming and often were subjected to various inadvertent errors during manual adjustments.